A liquid crystal display comprises a liquid crystal cell, a polarizing plate and an optical compensatory sheet (phase retarder). In a display of transmission type, two polarizing plates are placed on both sides of the liquid crystal cell, and the optical compensatory sheet is provided between the cell and one or each of the polarizing plates. On the other hand, a display of reflection type comprises a reflection plate, a liquid crystal cell, one optical compensatory sheet and one polarizing plate, piled up in this order.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates, and the electrode layer has a function of applying a voltage to the rod-like liquid crystal molecules. According to alignment of the rod-like liquid crystal molecules in the cell, various display modes are proposed. Examples of the display modes for transmission type include TN (twisted nematic) mode, IPS (in-plane switching) mode, FLC (ferroelectric liquid crystal) mode, OCB (optically compensatory bend) mode, STN (super twisted nematic) mode and VA (vertically aligned) mode. Examples of the modes for reflection type include HAN (hybrid aligned nematic) mode.
The polarizing plate generally comprises a pair of transparent protective films and a polarizing membrane provided between them. For preparing the polarizing membrane, a polyvinyl alcohol film is soaked with aqueous solution of iodine or a dichromatic dye, and is then monoaxially stretched.
The optical compensatory sheet is generally provided in various liquid crystal displays, to prevent the displayed image from undesirable coloring and to enlarge a viewing angle of the liquid crystal cell. As the optical compensatory sheet, a stretched birefringent polymer film has been conventionally used.
Recently, in place of the stretched birefringent polymer film, an optical compensatory sheet comprising a transparent support and a thereon-provided optically anisotropic layer formed from liquid crystal molecules (particularly, discotic liquid crystal molecules) has been proposed. The optically anisotropic layer is formed by aligning the liquid crystal molecules and then fixing the alignment. As the liquid crystal molecules, liquid crystal molecules having polymerizable groups are generally used. For fixing the alignment, they are polymerized. The liquid crystal molecules give large birefringence and have various alignment forms, and accordingly an optical compensatory sheet obtained from the liquid crystal molecules has a specific optical characteristic that cannot be obtained from the conventional stretched birefringent polymer film.
The optical characteristic of the optical compensatory sheet is designed according to that of the liquid crystal cell, namely, according to display mode of the liquid crystal cell. If an optical compensatory sheet is made with liquid crystal molecules (particularly, discotic liquid crystal molecules), various optical characteristics can be realized according to the display mode of the liquid crystal cell.
Various optical compensatory sheets using discotic liquid crystal molecules have been proposed according to liquid crystal cells of various display modes. For example, the optical compensatory sheet for liquid crystal cell of TN mode is described in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620. The compensatory sheet for liquid crystal cell of IPS or FLC mode is described in Japanese Patent Provisional Publication No. 10(1998)-54982. The compensatory sheet for OCB or HAN mode is described in U.S. Pat. No. 5,805,253 and International Patent Application No. WO96/37804. The compensatory sheet for STN mode is described in Japanese Patent Provisional Publication No. 9(1997)-26572. The compensatory sheet for VA mode is described in Japanese Patent No. 2,866,372.
The optical compensatory sheet comprising liquid crystal molecules may be laminated on the polarizing plate to form an elliptically polarizing plate, in which the optical compensatory sheet can serve as a transparent protective film on one side of the polarizing plate. The thus-formed elliptically polarizing plate has a layered structure in which a transparent protective film, a polarizing membrane, a transparent support and an optically anisotropic layer formed from liquid crystal molecules are piled up in this order. Since one element plays two roles (namely, since the transparent support serves as both of a support of the optical compensatory sheet and a protective film of the polarizing plate) to simplify the structure, the display can be made thinner and lighter. Further, in preparation of that display, the step of assembling the reduced element (one protective film of the polarizing plate) can be omitted and hence troubles in assembling can be reduced. The above unified elliptically polarizing plate, in which the transparent support serves as both of a support of the optical compensatory sheet and a protective film on one side of the polarizing plate, is described in Japanese Patent Provisional Publication Nos. 7(1995)-191217, 8(1996)-21996 and 8(1996)-94838.
However, a liquid crystal display equipped with the optical compensatory sheet or the aforementioned unified elliptically polarizing plate often gives an image with fine defects. The applicants have studied the cause of that, and found that uneven thickness of the transparent support induces the defects.
In preparing the optical compensatory sheet comprising a transparent support, an orientation layer and an optically anisotropic layer made of fixed liquid crystal molecules, it is necessary to fix closely the orientation layer (normally, made of polyvinyl alcohol) on the transparent support (normally, made of cellulose ester such as cellulose acetate). The affinity between cellulose ester and polyvinyl alcohol is, however, so poor that the interface is easily cracked or broken, and hence an undercoating layer made of gelatin is generally provided on the cellulose ester film to increase the adhesion between the support and the orientation layer. For forming the undercoating layer, a coating solution is prepared and applied on the cellulose ester film. Since the undercoating layer must be fixed well on the cellulose ester film, a solvent (for example, a ketone) soaking well into the cellulose ester film is used as the solvent of the coating solution. However, if the solvent soaks enough to swell the cellulose ester film, the film often shrinks and unevenly winds when it is dried. If the orientation layer and the liquid crystal layer (optically anisotropic layer) are formed on the winding film, the formed orientation layer has uneven thickness and the liquid crystal molecules in the optically anisotropic layer are unevenly aligned according to the winding. Consequently, the resultant liquid crystal display gives an image of poor quality.
In place of providing the undercoating layer of gelatin, the cellulose ester film may be immersed in an aqueous alkaline solution to improve the adhesion between the film and a hydrophilic layer (e.g., the orientation layer). This is generally known as “saponifying bath method”, which is described in Japanese Patent Provisional Publication No. 8(1996)-94838. Both top and bottom surfaces of the cellulose ester film are made hydrophilic at the same time by the saponifying bath method, and hence if the film is wound up into a roll after the hydrophilic layer (for example, made of polyvinyl alcohol) is provided on one surface, the hydrophilic layer (which is provided on the top surface) often sticks to the bottom surface (on which the hydrophilic layer is not provided) in the roll. For avoiding this trouble, the surface not to be saponified can be masked and then the film can be immersed in the alkaline solution to saponify only one surface of the film. However, since not only such complicated procedure is additionally needed but also additional wastes are produced, that process is not preferred in consideration of productivity and environmental preservation.
To solve the problem, it is proposed another saponification process comprising the steps of: continuously applying an alkaline solution comprising water (and an organic solvent) onto only one surface of the polymer film on which the orientation layer is to be provided, leaving the film to let the reaction proceed, and washing away the alkaline solution from the film.
However, although some organic solvents neither dissolve nor swell the polymer film, they often abstract additives such as plasticizers from the film. Since most of the additives are hydrophobic, they precipitate while the alkaline solution is diluted when the saponified film is washed with pure water. The deposited precipitates are attached on the film surface, and scatter light to cause haze. As a result, the film gives high haze, and accordingly has poor quality as an optical film.